1. Field of the Invention
This invention relates to a novel microemulsion utilizing as the surfactant a C.sub.8-16 orthoxylene sulfonate and a method of recovering crude oil from subterranean formations utilizing such microemulsion. More specifically, the present invention preferably relates to such microemulsion and method of recovering crude oil therewith in a secondary or tertiary recovery method wherein the microemulsion is injected into a subterranean formation so as to displace the crude oil therein.
2. Description of the Prior Art
The crude oil which is accumulated in subterranean formations is recovered or produced therefrom through one or more wells drilled into the subterranean formation with the initial production of the crude oil being carried out by what is referred to as "primary recovery," i.e. where only initial formation energy is used to recover the crude oil. The primary recovery technique, however, leaves substantial quantities of crude oil in the subterranean formation. Accordingly, the recognition of the large amount of crude oil in many oil-producing subterranean formations and reservoirs has led to the use of so-called "secondary and tertiary recovery" techniques which have as their primary purpose the economical recovery of additional quantities of oil known to be present in the subterranean formation or reservoir after primary depletion.
Probably one of the most common secondary recovery techniques is the so-called "waterflooding" in which aqueous fluids are injected at one or more points in the reservoir at pressures sufficient to be forced out into the reservoir and toward a spaced production well or wells so as to effect a displacement of the oil from the pores of the reservoir and a driving of the oil ahead of the water front to the production well or wells.
As can be easily understood the waterflooding technique and similar secondary recovery techniques are only advantageous when the cost of the water and any additional necessary chemical modifiers is less than the value of the oil which is recovered after primary depletion. Accordingly, the displacement efficiency of the waterflood and similar techniques has been a determining factor in connection with whether such technique can be satisfactorily utilized in oil recovery.
The displacement efficiency of the water itself is relatively poor primarily due to the formation of discontinuous oil droplets from the continuous oil as the waterflood proceeds. Furthermore, there is a relatively high interfacial tension between the water and the oil which contributes to the capillary retention of the discontinuous oil and thereby prevents its displacement by water under pressure gradients feasible in reservoir flooding processes. The displacement efficiency decreases with increasing interfacial tension thereby making recovery of oil quite difficult.
As a result of the foregoing, various aqueous surfactant systems have been proposed for use in waterflooding processes for recovering oil. Thus it has been proposed that the interfacial tension between the oil and water can be reduced from a characteristic value of the order of 35 dyne per centimeter to a value of less than 1 dyne per centimeter with the proper selection of a surfactant or surfactants. Here again, however, the selection of a particular surfactant depends not only upon the ability of the surfactant to reduce the interfacial tension between the oil and water but in addition the cost of the surfactant, since again in any secondary or tertiary recovery technique the cost of the materials utilized must be significantly less than the value of the oil which can be recovered.
Not only have surfactants been proposed for utilization in secondary recovery techniques such as waterflooding but in addition aqueous surfactant solutions have been proposed for tertiary recovery techniques to recover residual oil from a "watered-out" reservoir. In such uses a slug of an aqueous surfactant system will be introduced into the watered-out reservoir followed by a driving fluid to drive the slug of aqueous surfactant through the reservoir so as to allow displacement of the residual oil trapped in the pores of the reservoir.
More recently, secondary and tertiary recovery techniques have turned toward the applicability of microemulsions, the microemulsions comprising an oil, i.e. a refined or crude oil, an aqeous medium, sufficient surfactant to form the microemulsion, and optionally an electrolyte and cosurfactant. Such microemulsions have advantages when compared with aqueous surfactant solutions in that in the microemulsion there is a lessened tendency for the surfactant to be retained by the formulation rock, thereby allowing a more efficient utilization of the surfactant for displacement of the crude oil in the secondary and tertiary recovery techniques and an associated increase in oil recovery. Furthermore, microemulsions show good injectivity and almost complete recovery of the crude oil while intact.
While various types of microemulsions have been developed for secondary and tertiary recovery techniques, the microemulsions themselves have certain disadvantages including problems of stability and salt tolerance, with the surfactants developed to date being those which are generally useful under conditions of low salinity only. Noting these problems, the art has long sought a microemulsion system which is stable and can effectively and economically provide for the displacement and recovery of crude oil in secondary and tertiary recovery techniques eliminating the deficiencies associated with aqueous surfactant solutions and previously developed microemulsions.